The basal alveoli drain first during the exhalation, until the closing capacity is reached and the apical alveoli are the only ones left to drain. The reason this occurs is because the basal alveoli are more compliant than the apical alveoli, and as a result receive more oxygen during the initial breath of 100% oxygen, so the concentration of nitrogen is actually higher in the apical alveoli, as less of the nitrogen is washed out in that initial breath. In phase 4 there is an inflection and a sudden increase in the nitrogen concentration, and this occurs at the closing capacity. In phase 3 there is a plateau, representing only alveolar ventilation, as all dead space gas has now been exhaled. Towards the end of this phase, most of the dead space oxygen will have already been exhaled, and more alveoli will be now be draining their nitrogen and therefore the nitrogen concentration detected increases. This means that some alveoli will empty their nitrogen before others, so there will be mixing of alveolar and dead space gas. In phase 2 you’re detecting a gradually increasing concentration of nitrogen, because different alveoli have different time constants. To begin with, pure oxygen is exhaled from the dead space, so no nitrogen is detected - this is phase 1. The concentration of nitrogen detected is plotted against volume to look like the graph below, where Area A = Area B. Patient exhales all the way to residual volume into a pneumotachograph, which measures flow over time and therefore provides a volume measurement. Patient takes a vital capacity breath of pure (100%) oxygen, thereby removing all nitrogen from the anatomical dead space ( Remember that the alveoli still receive nitrogen from the blood). This works fairly well as long as you remember that this assumption will usually over estimate dead space, as arterial CO2 is slightly higher than alveolar, and will also be affected by:Īnatomical dead space is measured using Fowler’s method. This assumption is that the alveolar partial pressure of CO2 can reasonably be approximated by the arterial partial pressure of CO2, which makes it all a lot easier to measure. The Enghoff modification simply extends this further with another assumption. Therefore we can generate the equations below, and rearrange as demonstrated to form the Bohr equation. Therefore the entire expired CO2 is only going to be coming from the alveolar ventilation. We can reasonably assume that a tidal volume is comprised of alveolar volume and dead space volume, and we can also reasonably assume that inspired CO2 is minimal, if there is no rebreathing occurring. It is usually around 200-350ml in normal tidal breathing. Total or physiological dead space is measured using Bohr’s equation. This can be physiological, such as in hypoxic pulmonary vasoconstriction, or pathological, as seen in pulmonary embolism.Alveolar dead space refers to the alveoli that are ventilated but do not receive enough blood to undertake gas exchange.Anatomical dead space refers to the volume occupied by the conducting airways that supply the alveoli, but don’t undertake gas exchange themselves, and this is generally the first 16 airway generations.Physiological dead space = Anatomical dead space + Alveolar dead space Anatomical dead space Likewise if you have an adult patient on a ventilator and they're only achieving tidal volumes of 200ml, it's likely that most of that is dead space ventilation, and they're likely to become steadily more hypoxic and hypercapnoeic. Why does it matter?īecause if you take lots of rapid shallow breaths, you will be moving air in and out of your dead space, without sending any meaningful ventilation to the areas of your lung that can engage in gas exchange. It is composed of anatomical and alveolar dead space. The total dead space is called the physiological dead space. These areas therefore do not undertake gas exchange with the blood. If the air and the blood are being sent to different alveoli, then clearly the system isn't going to work very well, and this is called a ventilation/perfusion mismatch.ĭead space refers to the areas of the respiratory tract that are ventilated but not perfused. Not only does the air need to be drawn successfully into the alveoli, but blood needs to be pumped to those same alveoli in order to pick up the oxygen. The lungs represent an interface between ventilation and perfusion. This is why you start assessing your patient with an airway, breathing, circulation approach, because that's the most life-threatening order in which problems present. To survive, we need to get oxygen from the air into our blood, in order to supply the tissues.
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